A polyphase synchronous electric machine, for example a variable-reluctance motor, comprises (see FIG. 1) a rotor 1, a permanent magnet or equivalent, integral with a spindle 2 controlled in angular rotation, and a stator 3 having a plurality of pair-wise opposite salient poles 4 corresponding to the phases of the machine. The ferromagnetic structure of the stator comprises coils 5 supplied with electric current to induce a magnetic field orienting the rotor.
The electric current supply to the coils, generally a battery 20, whose output is filtered by a filter 30 delimited by a discontinuous closed line comprising a choke 350 and a capacitor 360, is controlled separately in each coil by virtue of a synchronous rectification device comprising an H-bridge 10 situated between the battery and the earth so as to create a rotating field driving the rotor at the same so-called synchronism speed.
The H-bridge 10 delimited by a discontinuous closed line comprises two parallel electrical links 11 and 12 forming the vertical branches of an H, each link 11, 12 comprising two electronic switches 21, 31 and 22, 32, separated by a midpoint 13 or 14, the midpoints 13 and 14 of these links being connected together by the coil 5 of a phase of the machine.
In a contemporary embodiment of the H-bridge, generally adopted by reason of its simplicity, the switches may be transistors 23, 34, and diodes 33, 24, as in the example of FIG. 1, the diodes and the transistors being situated on a diagonal.
The transistors 23 and 34 are instructed by an electronic circuit 40 to allow the passage of the current through the coil 5 while the diodes 33 and 24 act as spontaneous switches making it possible to absorb the energy accumulated in the said coil. This manner of operation is termed asynchronous.
However the diodes, which exhibit significant leakage currents, bring about a not inconsiderable energy loss, even when stopped, and the overall efficiency of the machine may find itself limited to 85%.
Moreover, the switches are previously determined so as to always be instructed or always be “spontaneous”, and the energy losses in the H-bridge turn out to be unbalanced, which hinders the reliability of the machine.
An alterno-starter consists for example of a variable-reluctance synchronous machine comprising an H-bridge and able to work in generator and motor modes, depending on the synchronous and asynchronous instructions which are imposed successively on the switches 21, 22, 31, 32.
Instructed as a generator, it transforms part of the mechanical energy available on the motor shaft into electrical energy to supply the electrical installation of the vehicle and recharge the battery through the diodes, hence in asynchronous mode.
Instructed as a motor, it transforms the electrical energy available on the battery, into mechanical energy either to start the engine of the vehicle, in which case it carries out the starter function, or to help it when cold, or to execute the so-called “stop and go” function of frequent stops and restarts, especially in town.
In the latter case, it is above all the transistors, instructed in synchronous mode, which transmit the energy from the battery to the coils of the stator, occasionally, and over short durations.
This synchronous machine has to be very reliable.